There might be a better subforum to ask the questions than this one, but I won't hold it against you.
There is no single answer to your questions, though.
A breakout board with separate drivers has some advantages - for example, if something goes wrong with one axis, it may be easier to troubleshoot or repair than it would be if you were using an all-in-one solution.
On the other hand, a combination board that has the breakout functionality built in may be less expensive than using separates.
As far as the specific combination board that you are considering, some people have had success with those while others have had problems. There is a lot of good information (but it's a long read) in the following thread:
A problem with keeping things inexpensive to start is that you may "grow out of" your first system quickly, so that in the longer run you end up spending more money than if you had gone with a more expensive set-up to start with.
I did not find any info about the stepper motors that you mentioned, but my guess is that they are not very powerful. Do you know what the holding torque rating of those motors is? What kind of mini mill are you contemplating modifying for CNC?